1. Field of the Invention
The present invention relates to a solid electrolytic capacitor that includes a lead frame, and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, a solid electrolytic capacitor (1) having the configuration shown in FIG. 8 is known (see Japanese Patent No. 3157722). The solid electrolytic capacitor (1) includes a capacitor element (2) with a thin wire-like anode lead (20) protruding therefrom. An anode lead frame (3) is resistance welded to the anode lead (20), and a cathode lead frame (30) is attached to the peripheral surface of the capacitor element (2) by a conductive adhesive (26). The capacitor element (2) and base ends of both lead frames (3) and (30) are covered with a synthetic resin housing (5), while portions of the lead frames (3) and (30) that protrude outside the housing (5) are bent downward along the peripheral surface of the housing (5), and then horizontally along the base of the housing (5).
When forming the solid electrolytic capacitor (1), the capacitor element (2) with the lead frames (3) and (30) attached is deployed in a space (91) between removable upper and lower dies (9) and (90), as shown in FIG. 9. After injecting resin into the space (91), the dies (9) and (90) are removed to form the housing (5) covering the capacitor element (2). Tapered surfaces (5a) (see FIG. 8) are formed on lower side portions of the housing (5) due to the removal of the dies. The lead frames (3) and (30) are then bent downward along the peripheral surface of the housing (5).
The capacitor element (2) is obtained by sequentially forming a dielectric oxide film (21), a solid electrolytic layer (22) made of a conductive polymer, and carbon and silver paste layers (23) on the surface of an anode body (24) composed of a sintered valve metal such as tantalum to cover the anode body (24), as shown in FIG. 10. The anode lead (20) is made of a valve metal, and protrudes from the center of the anode body (24).
As shown in FIG. 8, the anode lead frame (3) forms a step portion (7) within the housing (5). The step portion (7) includes a first horizontal portion (41) positioned higher than and substantially parallel to the anode lead (20), and a second horizontal portion (40) contacting the anode lead (20). The anode lead frame (3) is bent from the end of the first horizontal portion (41) along the outside of the housing (5) to form a vertical portion (35).
Due to variation in length of the anode lead (20), the leading end (20a) of the anode lead (20) sometimes comes in close proximity to an intersection X between an extended surface of the anode lead (20) and the housing (5).
By positioning the upper end of the vertical portion (35) above the second horizontal portion (40) by the height of the step portion (7), room is provided between the intersection X and the vertical portion (35) by an amount equal to the horizontal distance of the tapered portion (5a) corresponding to the height of the step portion (7). Damage to the leading end of the anode lead (20) due to the vertical portion (35) contacting the leading end of the anode lead (20) when bending the anode lead frame (3) along the housing (5) to form the vertical portion (35) is thereby prevented.
The applicant was aware of the following problems with the above configuration in need of resolution.
Springback sometimes occurs because of the step portion (7) being formed by bending the metal anode lead frame (3). Here, springback indicates the deformation of the step portion (7) in such a way that the angle of bend widens after the metal has been bent, as shown by the angles Y and Z magnified in FIG. 11. Consequently, the anode lead (20) of the capacitor element (2) is attached to the second horizontal portion (40) of the anode lead frame (3) at an angle, as shown in FIG. 12, possibly causing variation in the size of the contact area between the anode lead (20) of the capacitor element (2) and the anode lead frame (3).
Also, if the anode lead (20) is welded to the second horizontal portion (40), as shown in FIG. 13, the capacitor element (2) welded at an angle pushes the cathode lead frame (30) upward, possibly resulting in the cathode lead frame (30) jutting out from the housing (5) and affecting the appearance of the solid electrolytic capacitor (1). Variation in the bonding strength of the capacitor element (2) occurs as a result, and reliability is brought into question.
Also, the following problems occur if the positional relation between the anode lead (20) and the anode lead frame (3) is out of alignment when the capacitor element (2) is attached to the cathode lead frame (30). The substantial contact area between the anode lead (20) and the anode lead frame (3) is narrowed because of the anode lead (20) and the anode lead frame (3) not being parallel. This results in problems such as weak bonding strength and increased ESR (Equivalent Series Resistance).
Further, if the second horizontal portion (40) is formed on a large scale with a die when mass producing the anode lead frame (3), the second horizontal portion (40) becomes rounded due to wearing of the die, possibly reducing the contact area with the anode lead (20).
There are also calls for further reductions in the size and increases in the capacity of capacitors of this type.